Atmospheric turbulence is characterized by extremely high Reynolds numbers, which implies a very large range of scales present in the flow. The largest scales are on the order of 1 km and the smallest on the order of 1 mm. Conducting experiments in such conditions is very complicated since all scales need to be resolved. This implies long sample times, at high sample rates using small sensors. The size of the sensor needs to be on the order of the smallest scales, or smaller, and the bandwidth needs to be higher than the frequency corresponding to that of the smallest eddies. A windy day the wind speed typically is 10 m/s which, according to Taylor's frozen field hypothesis, will result in frequencies up to 10 kHz. Traditionally the bandwidth for sensors used in the atmosphere are much lower than that, but efforts have been made to resolve the complete frequency spectrum for the turbulent velocity field with fast-response velocity sensors, that can survive the rough conditions they are exposed to in the atmosphere.
In order to accurately predict the energy balance at the earth's surface, a critical component to any weather prediction or climate model, one needs information about the scalar fields in addition to the velocity field. The scalar fields of interest are mainly temperature and humidity, since those constitute the main contributions of heat fluxes away from the surface due to the atmospheric flow. The sensible heat flux is the covariance between the temperature field and the velocity component normal to the surface, ω′θ′, which corresponds to the energy transferred away from the surface, in form of temperature. The latent heat flux is the covariance between the same velocity component and the humidity field, ω′q′, which is the part of the energy carried into the atmosphere by evaporation of water at the surface. The ability to predict these fluxes will allow closure to the governing equations, which is the purpose of turbulence models.
Unfortunately, the two covariances are very challenging to measure experimentally, and only the equations for very low Reynolds can be solved numerically. Two methods are commonly used to measure humidity in the air: laser based and capacitance based sensors. A fast response laser based system is typically too expensive to densely instrument test sites (even the conventional slow response systems are very expensive). Capacitance based systems will always have a time response several order of magnitudes too slow, since it takes time to replenish the cavity between the electrodes.
Another method to measure humidity is to measure the thermal conductivity of the air, which is a function of humidity. Sensors based on this technique have been tested and shown to work well. Unfortunately, distinguishing the sensitivity of humidity from air velocity is difficult using this method, since both act to increase the heat transfer from a heated element. The currently available techniques for measuring humidity are neither fast nor small enough to capture small scale turbulent fluctuations.
In addition, in some fields, accurate measurement of low humidity levels is critical, such as in the natural gas industry. There, water vapor must be removed from the gas stream in order to prevent problems relating to the processing, storage, and transportation of natural gas—water can lead to corrosion and/or the formation of hydrates. Additionally, there are often statutory or contractual limits to the water vapor concentration in gases; in the United States the maximum absolute humidity for interstate transfer of natural gas is set at 7 pounds per million standard cubic feet. Further due to the explosive or flammable nature of some gases, some measurement techniques cannot be utilized. The sensors that have been developed are therefore typically either cannot be used in hazardous areas, are very expensive, have significant response times, and/or are subject to drift over time.
Therefore, there is an acute need for fast-response, small-size, humidity sensors that can be used in the field over a broad range of humidity and temperature.